1) Field of the Invention
The present invention relates to an optical reception circuit and a controlling method for the same, and particularly to a technique suitable for the improvement, for example, in the reception sensitivity characteristic of an optical receiver used for optical communication.
2) Description of the Related Art
FIG. 11 is a block diagram showing a configuration of essential part (main signal unit) of an optical receiver. The optical receiver (main signal unit 100) shown in FIG. 11 includes a photoelectric converter 110 having a light reception device 111 such as an avalanche photodiode (APD), a pre-amplifying unit 120 having a preamplifier (output differential amplifier) 121, and a main amplifier 130 having a limiter amplifier (differential amplifier) 131. In the optical receiver (main signal unit 100), light received by the light reception device 111 is converted into a current signal corresponding to a light amount (light intensity) of the received light. Then, the current signal is converted into a voltage signal of a value corresponding to that of the current signal and amplified to a required voltage level by the preamplifier 121. The amplified voltage signal is outputted as a differential voltage signal from a non-negated (positive phase) output terminal and a negated (negative phase) output terminal of the preamplifier 121 to the limiter amplifier 131.
In the limiter amplifier 131, the reference voltage (identification voltage) is adjusted (fixed) to a time average value (that is, identification level=50%) of the amplitude of the differential voltage signal (bipolar sign) so that the limiter amplifier 131 identifies and regenerates the data. In other words, a magnitude comparison result between the reference voltage and an input voltage signal is outputted as a data identification result.
Incidentally, the amplitude of the output voltage of the amplifiers 121 and 131 in the optical receiver is likely to be varied by dispersion in fabrication or an operation condition (temperature variation and so forth) of them. Therefore, it is preferable to automatically control the identification level to the time average value of the amplitude of the differential voltage signal. Therefore, also a technique is known wherein, for example, as shown in FIG. 12, an identification level controller 140 is provided so that the reference voltage for the limiter amplifier 131 is automatically adjusted (controlled) to the time average value of the amplitude. The automatic adjustment is implemented, for example, by monitoring the output voltage of the limiter amplifier 131.
It is to be noted that, as another known technique, a technique proposed in Japanese Patent Laid-Open No. HEI 9-270755 (hereinafter referred to as Patent Document 1) is available. The technique relates to an automatic threshold value controlling circuit used in an optical reception apparatus, and it is an object of the technique that, even if an offset voltage for malfunction prevention is applied, a threshold value voltage (which corresponds to the reference voltage mentioned hereinabove) is controlled to a time average value of the amplitude of reception data so that, even if the duty ratio of the reception data is degraded by a non-linear characteristic of a preamplifier, the degraded duty ratio can be corrected.
To this end, in the technique disclosed in Patent Document 1, where the amplitude of the reception data is in a linear region of the preamplifier, the identification voltage is controlled to the time average value of the amplitude of the reception data. However, where the amplitude of the reception data is in a non-linear region of the preamplifier, the identification voltage is controlled so as to be higher than the time average value of the amplitude of the reception data.
Also a technique proposed in Japanese Patent Laid-Open No. HEI 6-310967 (hereinafter referred to as Patent Document 2) is available as a further known technique.
This technique relates to an identification level controlling method for a system wherein the reception signal level in burst reception or the like varies by a great amount in time. In such a system as just described, it is necessary to control the identification level dynamically depending upon the reception signal level. Therefore, the system includes a peak detection circuit for detecting a peak level of a reception signal and a bottom detection circuit for detecting a bottom level of the reception signal, and outputs a substantially middle value between a peak value and a bottom value as a threshold value.
As a result of increase of data traffic in recent years, high-speed and great capacity communication is essentially required. Construction of a great capacity photonic network using high density wavelength division multiplexing (Dense Wavelength Division Multiplexing) is proceeding. The form of a network varies from a point-to-point type to a ring or mesh type, and a complicated configuration is applied wherein the path and the transmission distance differ depending upon the wavelength. If the transmission distance differs, then also the influence of chromatic dispersion on the transmission light differs.
For example, the optimum identification level differs in response to the chromatic dispersion amount as seen in FIG. 5. It can be recognized that, where the identification level is fixed to 50%, the BER degrades significantly from 1E-12 to 1E-6 at a positive dispersion of 1,600 ps/nm. Therefore, in an optical receiver, it is necessary to absorb the influence of the chromatic dispersion so as to satisfy a desired reception sensitivity characteristic. However, while various controlling methods of an identification level including the techniques disclosed in Patent Documents 1 and 2 have been proposed as a countermeasure for improving the reception sensitivity characteristic of an optical receiver, a method for suppressing degradation of the reception sensitivity caused by an influence of chromatic dispersion has not been proposed as yet.